An improved incremental conductance method with a variable step size is proposed to solve the problem that the traditional maximum power point tracking (MPPT) algorithm cannot strike a balance between the tracking speed and steady-state oscillation. The MPPT speed can be increased by the improved incremental conductance method using a zonal variable step size. Meanwhile, the steady-state oscillation problem is optimized by using the incomplete partial differential theory, thus improving the efficiency of photovoltaic power generation. The feasibility and effectiveness of the improved incremental conductance method is verified by comparing the traditional control algorithm with the improved incremental conductance method.

Aimed at the problem of eddy current loss, a method for the parameter design and optimization of a wireless power transfer (WPT) system in seawater is proposed to optimize the power transmission efficiency of the system. First, based on the analysis of the electromagnetic field of coils under operation in seawater, the equivalent mutual inductance model of the WPT system in a marine environment is obtained by using the equivalent impedance of eddy current loss. Second, when the positions of the primary- and secondary-side coils are fixed, the corresponding relationship between the equivalent impedance of eddy current loss and the operating frequency of the system and the number of coil turns is established, and the feasibility of the calculation method for the equivalent impedance of eddy current loss is verified by using the coils on both sides of the WPT system. Finally, based on the energy model of an LCC/S-type WPT system in seawater, the particle swarm optimization algorithm is used to optimize the transmission efficiency. A test system was built with the optimized parameters, and results show that when it transmitted 1 kW of power in a simulated marine environment, its overall efficiency can reach 84%.

To enhance the accuracy of a lithium battery model and realize accurate state estimation of the lithium battery, a fractional-order electric model of the lithium battery was built, and the model parameters were identified using adaptive genetic algorithm. Based on the fractional-order electric model, the unscented Kalman filter was adopted to estimate the lithium battery’s state-of-charge (SOC) and state-of-health (SOH). Stimulation results show that, the established fractional- order electric model of the lithium battery can more accurately describe its dynamic characteristics during charging and discharging, and the accuracy of the proposed state estimation strategy was higher than that the conventional control strategy.

Since DC bias is one of the main reasons for increases in the vibration and noise of a large-scale transformer, it is essential to fully understand the vibration and noise characteristics of large-scale transformers under DC bias for the evaluation of the operating state of transformers and the reduction of noise and vibration. A 406 MVA EHV large-scale transformer is taken as the research objective, and its vibration and noise characteristics are studied. First, based on the field-circuit coupling finite element method, the no-load operation characteristics under different DC bias currents are simulated and analyzed, and the law of excitation current under different DC biases is analyzed. Second, a multi-physics coupling model of circuit-magnetic field-solid mechanics-pressure acoustics is established, and the effective value of vibration displacement and the time-frequency characteristics of noise signal at different measuring points of the transformer under DC bias are obtained considering the influence of magnetostriction. Third, the sound level is measured at different measuring points around the transformer, and the simulated value is compared with the actual measured value to verify the effectiveness of the proposed calculation method for transformer vibration and noise. Finally, the Hilbert-Huang transform method is used to extract the vibration and noise characteristic quantities of one large-scale transformer under DC bias, and a transformer vibration characteristic recognition method based on the energy ratio of the noise signal intrinsic mode function is proposed. This method can effectively recognize the severity of DC bias of the transformer and accurately grasp its operating state, providing a theoretical basis for timely taking measures to suppress the DC bias.

Aimed at the problems of DC bias and frequency variation in a weak grid, a modified inverse Park transform phase-locked loop (IPT-PLL) technology suitable for single-phase grid-connected inverters is proposed. First, the α component after Park transform is used as a reference voltage in the phase detector to solve the problem of DC bias in grid voltage, and an orthogonal component is constructed by the method of 1/4 fundamental periodic delay. Second, the fractional-order delay is approximated by Lagrange interpolation polynomial to reduce the calculation error of delay caused by frequency variation, and the design method for PI regulator is theoretically analyzed. Finally, experimental results show that the modified IPT-PLL proposed has a strong frequency adaptivity, and it can significantly suppress the interference of DC bias in grid voltage. In addition, its dynamic and static performances are satisfying.

Aimed at the transient instability of a voltage source converter (VSC) based on phase locked loop (PLL) under weak network conditions and considering the influences of power grid line impedance, VSC reactive power injection and PLL filtering, the transient instability boundary of VSC is comprehensively studied based on the critical voltage in a variety of fault scenarios such as grid voltage sag, frequency fluctuation and three-phase asymmetric fault. Through the analysis of the VSC grid-connected vector diagram in different operation scenarios, the mathematical models of relationships between the grid voltage of VSC grid-connected system and factors (e.g., line impedance and impedance angle, VSC operation power and its power factor, PLL phase-locked error, and grid frequency) in multiple scenarios are established. Then, the transient instability boundary of VSC is indicated based on the critical voltage. Results show that both the line resistance and reactive power injection can directly reduce the critical voltage and improve the stability of the system. The increase or decrease in grid-side frequency will directly lead to the phase lag or lead of PLL and an increase or decrease in line reactance, thus indirectly affecting the critical voltage and further affecting the transient stability of VSC. The PLL pre-filter may cause errors in the phase-locked result, and its phase lag or lead will reduce or increase the critical instability voltage of the system, respectively.

Phase-shifted full-bridge zero-voltage zero-current switching(ZVZCS) converters are favored in high- power DC conversion applications owing to their advantages such as simple structures and high efficiency. However, high-power phase-shifted full-bridge ZVZCS converters still face problems including difficulty in the current reset and severe duty cycle loss. In response to the above issues, a novel phase-shifted full-bridge ZVZCS converter is put forward, which ensures that it can realize zero-current switching over a wide load range by introducing an auxiliary circuit on the primary side to reset the current to zero before the turn-on of lagging-leg switches. At the same time, it can accelerate the commutation speed on the primary side, reduce the duty cycle loss and realize an optimized design of power supply. Based on the analysis of the circuit structure, working principle and characteristics of the proposed converter, a 1 kW experimental prototype was designed to verify its correctness.

Single-phase charging systems usually face the problem of secondary power pulsation. To solve this problem, a single-phase electric drive reconstructed onboard charger (EDROC) system with low voltage ripple is proposed, in which a Buck/Boost active filter is placed in parallel on the output side of a traditional single-phase EDROC system to absorb or compensate the secondary power pulsation, thereby obviously reducing the output voltage ripple of the charging system. First, the topology, working principle and secondary power pulsation generation mechanism of the single-phase EDROC system are analyzed. Second, a single-phase EDROC system with low voltage ripple is put forward by combing the Buck/Boost active filter. Meanwhile, the topology, working principle, and selection of inductors and capacitors of the Buck/Boost active filter are analyzed in detail. Third, a control strategy for the proposed EDROC system is designed. Finally, a 200 W experimental prototype was designed, and experimental results verified the feasibility of the proposed charger.

With the increasing penetration rate of renewable energy, carbon emissions are reduced. However, the inherent intermittency and volatility of renewable energy also bring problems such as inertia, security and economy to the power system. The battery energy storage(BES) technology has become one of the important means to solve this problem. Under this background, an autonomous control method for BES system oriented to the active support of grid voltage is proposed based on full-state feedback. First, based on sagging K_v(V_g-v_g) and the virtual capacitor C inertia technology, static power support control and dynamic voltage support control modules are designed, so that the BES system can provide power(static) support and voltage(dynamic) support. Second, the voltage controller and current controller are combined by using the full-state feedback method, which makes the design of the proposed controller more systematic and flexible and reduces the voltage oscillations caused by single-phase ground fault. Third, in order to maintain the stability of state-of-charge(SOC) of BES, a BES SOC controller based on regulatory factors is also designed to further improve the autonomous operation ca-pability of the BES system. Finally, a case study of a 14-node DC system was carried out based on MATLAB and a semi-physical simulation platform, and simulation results verified the effectiveness of the proposed method in the cases of double-support of static power and dynamic voltage and single-line ground fault. With this method, the BES system can be connected to any key node in the grid, and the voltage at the point of common coupling in the grid can be actively supported through the local monitoring of disturbance, which is not affected by disturbance and can be operated and controlled independently. In addition, this method can also prevent the converter from overcurrent during transient low-voltage accidents, so that the autonomous operation capability of BES is realized.

In the impedance measurement process, since the inverter impedance varied widely, the magnitude of injection disturbance cannot be evaluated in advance. Therefore, it is necessary to adjust the disturbance energy adaptively. The impedance measurement device of disturbance voltage injected in series is taken as the research object, and an adaptive adjustment strategy of disturbance voltage based on disturbance current feedback is proposed. The magnitude of disturbance voltage is adjusted by detecting the responding disturbance current in real time, thus realizing the adaptive adjustment of disturbance energy. Both the disturbance voltage and responding disturbance current are controlled to be within 10% of the steady-state point of the system under test. The effectiveness of the proposed control strategy was verified by hardware-in-the-loop simulations in real time.